This invention relates generally to an electrostatographic printing machine, and more particularly concerns an apparatus for controlling unfused image disturbing effects of a sheet motion opposing force during a sheet transport.
In an electrostatographic printing machine, a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing toner, for example, black toner, into contact therewith. This forms a toner powder image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is then separated from the photoconductive member and the toner powder is fed on the copy sheet through a fusing apparatus where it is heated to permanently affix it to the copy sheet, thus forming a black and white copy of the original document.
Multi-color electrostatographic printing which uses multi-colored toners is substantially identical to the foregoing process of black and white printing using only black toner. However, rather than forming a single latent image on the photoconductive surface, successive single color latent images corresponding to color separated light images of the original document are recorded thereon. Each single color electrostatic latent image is developed with toner particles of a color complimentary thereto. This process is repeated a plurality of cycles for differently colored images using their respective complimentarily colored toner particles to form color toner images. Each single color toner powder image is transferred to a copy sheet in superimposed registration with the other toner powder images. This creates a composite multi-layered toner powder image on the copy sheet. The copy sheet is separated from the photoconductive member and, thereafter, the multi-layered toner powder image on the sheet is fed through a fusing apparatus and permanently affixed to the copy sheet, thus creating a color copy of the original multi-color document.
In a black and white or multi-color electrostatographic printing machine, the copy sheet is typically brought into moving contact with the photoconductive member during toner powder image transfer to the copy sheet. A sheet transport apparatus is typically provided for receiving the copy sheet incrementally as it is incrementally separated from the photoconductive member, and for transporting the copy sheet towards and into the fusing apparatus. A typical fusing apparatus includes a rotatable heated roller and a rotatable back up roller which form a fusing nip for contacting, engaging and frictionally driving the copy sheet with the toner powder image thereon through the fusing nip.
In transporting the copy sheet into the fusing nip, there is a specific kind of toner powder image disturbance or smear that occurs only once, and only at a particular instance. This specific kind of image disturbance or smear is a copy defect that is created at the instance when the lead edge of the copy sheet contacts any part of the fusing apparatus. This is because when the lead edge of the copy sheet contacts the fusing apparatus, an instantaneous backward force is generated by the fusing apparatus in the copy sheet regardless of the speeds of the sheet or the fusing apparatus. This backward force as such is in opposition to the forward motion of the copy sheet. This opposition occurs only during a small, short period of time (t). During such time period (t) motion, the opposing force virtually brings the lead edge to a complete stop, all prior to driving engagement of the lead edge by the fusing nip. As a consequence, the whole sheet also virtually stops instantaneously during such period (t), thus causing slippage of the separating sheet against the photoreceptor, and resulting in the specific image disturbance or smear described above.
The specific kind of instantaneous image disturbance or smear described here is believed to be different from on-going image smear that occurs due to a mismatch between the velocities or speeds of the fusing nip and the photoconductive member. Such on-going image smear is of concern, of course, only after the lead edge of the copy sheet is already in driving engagement within the fusing nip. On the other hand, the specific kind of instantaneous smear or "fuser smear" being addressed by the present invention occurs at the instance of lead edge contact within the fusing apparatus. Such contact as can be appreciated occurs prior to driving engagement of the lead edge by the fusing nip, and, consequently, the "fuser smear" defect would appear to be independent of speed or velocity.
Conventionally, however, "fuser smear" and on-going smear have been treated as being caused by speed or velocity mismatches between the sheet and fusing apparatus. As such, various devices and schemes have been proposed for preventing smearing, by matching the speed or velocity of the fusing nip and that of the sheet being transported thereinto. For example:
U.S. Pat. No. 3,902,645 describes a machine which includes rolls between which a flexible sheet is passed. After passing from one section, the flexible sheet falls downwardly to form a loop, the other side of which passes upwardly into another section of the machine. A motor drives a roll which advances the sheet from one section to the other section. A pivotable plate contacts the lowermost region of the loop. The direction that the plate pivots depends upon the whether the loop is increasing or decreasing. The direction that the plate pivots matches speeds by controlling the speed of the motor advancing the sheet.
U.S. Pat. Nos. 4,017,065 and 4,058,306 disclose a vacuum support interposed between the fuser and the photoreceptor. When the lead edge of the copy sheet enters the fuser roll nip, the vacuum is turned off and a buckle forms in the sheet due to the speed mismatch between the fuser and the photoreceptor.
U.S. Pat. No. 4,561,581 describes a web accumulator positioned between a variable speed drive and an intermittent drive. A portion of a web in the accumulator is curved into a downward extending loop by a curved support and the force of gravity acting on the web.
U.S. Pat. No. 5,294,965 issued Mar. 15, 1994, (Xerox) discloses an oscillating prefuser vacuum sheet transport which has a pivoting downstream end for compensating for a velocity mismatch between a fuser roll and an image receiver. The pivoted end of the transport forms a controlled buckle in an image carrying sheet after the sheet contacts the fuser roll.
U.S. 5,166,735 issued Nov. 24, 1992, discloses a prefuser sheet transport system that includes a buckle sensor and is offset from a linear path to the fuser nip. A signal from the buckle sensor is used to control the speed of the fuser to a matching value.
U.S. Pat. No. 4,905,052 issued May 22, 1990, discloses a copier/printer that has an image transfer station and a fusing station operating at different speeds in order to create a buckle in a sheet. A sensor located therebetween senses the size of the buckle, and at predetermined sizes thereof, adjusts the speeds of the stations.
U.S. Pat. No. 4,905,052 issued Feb. 27, 1990, discloses apparatus for compensating for velocity mismatches between adjacent sheet transports including a first or upstream transport which advances the sheet faster than a second downstream transport. The apparatus further includes a pivotable plate between the adjacent transports that pivots away from the sheet when the sheet contacts the second or downstream transport, thus allowing the sheet to buckle.
U.S. Pat. No. 4,561,756 issued Dec. 31, 1985, discloses a transport system for a short prefuser paper path. Speed mismatch compensation is provided by intentionally driving the fuser roller nip at a mismatching speed to create a downward buckle. The transport system includes a multiple baffle arrangement which allows the downward buckle to form prior to entering the fuser nip. The downward buckle is supposed to absorb speed mismatches thereby preventing image smearing.